1. Field of the Invention
The present invention relates to an ultrasonic diagnosis apparatus for scanning a slice of an object to be examined with an ultrasonic wave and detecting a frequency change in the reflected ultrasonic wave (Doppler shift) so as to detect the direction and flow rate of a blood flow within the slice.
2. Description of the Related Art
As one of such ultrasonic diagnosis apparatuses, a B-mode Doppler flow-mapping (to be referred to as BDF) apparatus is available. The BDF apparatus is designed to output a color display of a blood flow image included in a tomographic image (B-mode image) in accordance with a blood flow direction and a blood flow rate. In this apparatus, an ultrasonic wave is radiated a plurality of times n (n.gtoreq.2) in each radiation direction (i.e., a scanning line or raster), a change in phase of each reflected ultrasonic wave with respect to a corresponding previous reflected ultrasonic wave is detected, the phase changes are averaged to obtain a Doppler shift frequency, and the Doppler shift frequency at each point on the scanning line is calculated. At the same time, the ultrasonic radiation direction is changed, and a predetermined slice is scanned by the ultrasonic wave. A tomographic image is obtained from the intensity of the scanned ultrasonic wave, and the Doppler shift frequency is color-displayed on the tomographic image. In general, a flow direction to toward an ultrasonic transducer probe is colored in red, a flow direction from the probe is colored in blue, and a disturbed flow is colored in green. In addition, a flow rate is represented by the saturation of a color. By using such a BDF apparatus, an abnormal state of a blood flow such as a regurgitation, a constriction, or a shunt can be observed in real-time.
Principles of Doppler shift detection will be described below. When an ultrasonic wave is radiated on blood flowing in a living body, the ultrasonic wave is scattered by flowing blood cells and the frequency fo of the ultrasonic wave is subjected to a Doppler shift so as to be changed by a frequency fd. Therefore, the frequency f of the received ultrasonic wave is given by f=fo+fd. In this case, the frequencies fo and fd have the following relationship: EQU fd=2V.multidot.cos .theta..multidot.fo/C (1)
where V is the blood flow rate, .theta. is the angle defined by the ultrasonic wave and the blood vessel, and C is the ultrasound velocity.
Accordingly, the blood flow rate V can be obtained by detecting the Doppler shift frequency fd.
There are upper and lower limits of detectable Doppler shift frequencies, and hence upper and lower limits of detectable blood flow rates are present. In the BDF apparatus, an ultrasonic wave is radiated at a predetermined rate, wherein the repetitive rate frequency fr of the ultrasonic wave is equal to a sampling frequency. Therefore, the detectable Doppler shift frequency fd is limited according to the sampling theorem as follows: EQU fd.ltoreq.fr/2 (2)
Therefore, the upper limit V.sub.max of measurable flow rate can be represented as follows: EQU V.sub.max =C.multidot.fr/(4cos .theta..multidot.fo) (3)
A frequency exceeding the upper limit fr/2 is detected as being decreased by fr. In a BDF image, a blood flow rate exceeding the upper limit is displayed with its flow direction being reversed. In addition, when frequency analysis of a received ultrasonic wave is performed by a fast Fourier transform (FFT), and the analyzed resulting waveform is to be displayed, a portion exceeding fr/2 is shifted downward by fr, and a so-called aliasing phenomenon occurs.
Furthermore, the lower limit of measurable flow rate is limited by a data length to be fetched. If a data length to be fetched (a length of sampling on each scanning line) is represented by T, and a data number (the number of times of irradiation of an ultrasonic wave in the same direction) is represented by n, a lower limit fd.sub.min of detectable frequency can be given by: EQU fd.sub.min =1/T=fr/n (4) EQU .BECAUSE.V.sub.min= C.multidot.fr/(2n.multidot.cos .theta.fo)(5)
In a BDF image, flow rates below this lower limit are displayed as achromatic portions. For this reason, a portion where a flow rate is low, e.g., a portion near the wall of the blood vessel, tends to be an achromatic portion. This degrades the resolution of the BDF image.
It is apparent from equation (5) that the lower limit of measurable flow rate can be decreased by decreasing the sampling frequency fr or increasing the data number n. However, if the sampling frequency fr is decreased, the upper limit of detectable flow rate is also decreased according to equation (3), and aliasing tends to occur. If the sampling frequency fr is decreased or the data number n is increased, image quality in the BDF apparatus is degraded for the following reason. In the BDF apparatus, the following relation is established: EQU F.multidot.n.multidot.m.multidot.(1/fr)=1 (6)
where F is a frame number, m is the total scanning line number for the B mode tomographic image, n is the above-mentioned data number, and fr is the above-mentioned rate frequency. The frame number F is the number of B-mode images per second. As the frame number F is increased, the BDF image looks more natural as a motion image. Normally, F is set to be 8 to 30. If m=32, fr=4 KHz, and n=8, F is 16. According to equation (6), if the data number n is increased or the rate frequency fr is decreased, the frame number F is decreased, and the image looks unnatural. Note that if the total scanning line number m is decreased, the resolution of the BDF image is decreased.
A technique for decreasing this lower limit of detectable flow rate is disclosed in "Method and System for controlling ultrasonic scanning sequence," U.S. patent application Ser. No. 07/423,713 filed on Oct. 18, 1989 and assigned as the present invention to the same assignee. According to this technique, instead of changing the radiation direction to an adjacent direction after an ultrasonic wave is radiated n times in each direction, the radiation direction is changed one by one after each radiation. When the radiation direction is sequentially changed from the first direction through an ith direction, the ultrasonic wave is radiated in the first direction again. When the ultrasonic wave has been radiated n times in each radiation direction, radiation is performed in the same manner as described above in directions from an (i+1)th direction to a (2i)th direction. Subsequently, this operation is repeated. With this operation, the ultrasonic wave is radiated in each radiation direction with a frequency of fr/i. That is, the sampling frequency for Doppler shift detection can be decreased without sacrificing other characteristic features
In the conventional apparatus, however, the value i is arbitrarily determined by an operator, and no proper method of setting the value i has been established. As described above, the sampling frequency is changed when the value i is changed. As a result, the detectable range of Doppler shift frequency also varies. For this reason, if the value i is too large, although the lower limit is decreased, the upper limit of detectable flow rates is also decreased Hence, aliasing tends to occur. If the value i is too small, the lower limit of detectable flow rates cannot be satisfactorily decreased.